I added NuGet package "Microsoft.Bcl.Async" to my Silverlight 5 project and Resharper seems to highlight these two non-existant errors:

1) "The return type of async method must be void, Task, or Task<T>" - method signatures with async keyword and Task or Task<T> as a return type.2) "Type 'System.Threading.Tasks.Task' is not awaitable" - any await statement

The project builds fine, but Resharper doesn't seem to recognize that the types are present and thus always highlights errors in the actual code editor, which is super annoying.